This invention relates generally to a heating system for multiple position plate heaters utilizing vapor condensing at the same pressure throughout the system. More particularly, the invention relates to a condensing steam heating apparatus for synthetic yarn processing, and process therefor, having a very high overall coefficient of heat transfer even when the yarn is under the yarn-contacting heating surface.
As is well-known, vapor condensing systems are more suitable than electrical heating systems where it is necessary to heat uniformly moving threadlines of yarn such as polyester by means of contact with a hot plate. This is particularly so when adjacent positions are operated at different heat loads on account of different threadline deniers, number of threadlines etc. and most particularly so when the heating surface is approximately vertical. For example the temperature profile of an electrically heated hot plate of a commercial texturizing machine was determined under various conditions and the results are shown in FIG. 1. The hot plate is 4 ft. long and 1 inch broad, has two threading grooves and a matte chrome finish, is in the vertical position and is enclosed by a door to prevent heat escaping and draughts from changing the temperature profile; and has uniform electrical heating (6 volt, 30 amps) along its length. Without any yarn running on the hot plate and the thermostat at the top of the plate set to 180.degree. C., it can be seen that the bottom of the hot plate is at about 110.degree. C., i.e. there is a 70.degree. C. temperature differential. With one threadline of yarn running downwards on the plate the temperature differential is reduced to 40.degree. C., and with two threadlines the temperature differential is reduced to 20.degree. C. By having non-uniform electrical heating along the length of the hot plate it is possible to marginally reduce the temperature gradient for a given set of fixed conditions (threadline speed, number of threadlines etc.), but such a system is still highly inflexible and the product made from a twin threadline process cannot be merged with the product from a single threadline process (e.g. caused by one of the threadlines breaking or running out) on account of its significantly different properties such as ability to absorb dye etc.
Various heating systems of the vapor condensing type have been devised, and include, for example, that disclosed in U.S. Pat. No. 3,177,931 which claims "In apparatus for modifying heat-settable yarns, the combination comprising closed means including a source of vaporized heating fluid substantially completely free of incondensable gases, and a plurality of tubular members for receiving said fluid from said source, said tubular members being disposed in close adjacent side-by-side relation and being inclined for drainage of condensate back to said source, a yarn guide conduit in the form of an open trough generally V-shaped in transverse section and extending along and seated directly upon said tubular members with opposite sides thereof respectively engaging said tubular members for being heated thereby and means for drawing said yarn lengthwise of said tubular means and conduit in contact with the latter for being heated thereby." However, it has hitherto been considered both unnecessary and undesirable in such condensing vapor systems to have the yarn under the heating surface, because of the greater temperature variability resulting from the increase in thickness of the condensation film between the vapor and the plate heating the yarn. Accordingly none of the prior art of which the applicant is aware discloses a vapor condensing hot plate with the yarn running under the hot plate, e.g. FIG. 6 of the forementioned U.S. Pat. No. 3,177,931.
The applicants have now discovered that there would be certain processing advantages in having the yarn run under the vapor heated hot plate. In particular, when hot plates several feet long are used on a multiposition machine it is very much easier for the operator of the machine to string-up the threadline on the underside of a plate inclined at, say, 30.degree. to the vertical than to string-up a machine which is similar except that it has vertical hot plate. Longer hot plates may be used and thereby higher processing speeds. Also heat losses are marginally reduced: according to "The Efficient Use of Steam" By Oliver Lyle (H.M. Stationery Office), Table XXVI on page 853, heat losses are roughly proportional to the square of the air velocity, and the convection air velocity adjacent to a vertical hot plate is greater than the convection air velocity for the underside of the same hot plate inclined to the vertical.